It is the kind of letter that might well lead to a distinctly uncomfortable conversation around the breakfast table: this month, between 20,000 and 30,000 German households received legal warnings for having viewed copyrighted pornographic films via the streaming website RedTube.com.

Initially, it had been assumed that a court error had led to the sending of the letters, which ask for the payment of a €250 fine to the Swiss media agent that claims to hold copyright for the films, including Amanda's Secret, Miriam's Adventure and Glamour Show Girls. But now the legal firm behind the warnings says it plans to look into more infringements on porn streaming sites in the coming year. "Redtube was more like a test balloon," Thomas Urmann of U+C told Welt am Sonntag newspaper.

Were the Bavarian law firm to succeed, it would set a worldwide precedent. In the past, streaming sites have been able to circumvent copyright law, as no copy of the original work is created. But U+C, which specialises in file-sharing cases, argue that viewing clips on streaming sites can constitute a proliferation of copyrighted material since a tiny copy of the file is created in the memory of their computer.

Urmann said the letter had elicited responses from hundreds of internet user households, some of them from irate wives and partners. "Most of the conversations, however, have been very factual and businesslike."

RedTube's Alex Taylor released a statement saying: "RedTube stands by its firm opinion that these letters are completely unfounded and that they violate the rights of those who received it in a very serious manner," describing the court's actions as blackmail and a violation of privacy.

The copyright lawyer Christian Solmecke told the Guardian that there was not only "no legal basis" for the fines, but that it was possible that the law firm behind the letters may have broken the law: "It is hard to imagine how the IP addresses of the users could have been obtained on a legal basis."

He suggests that the Cologne state court that handed out the names connected to the IP addresses to U+C law firm only did so in the mistaken belief that RedTube.com was a file-sharing site like BitTorrent. Furthermore, research by online news portal heise.de suggests that those users who received the warning were redirected to the copyrighted clips without their knowledge, in what the site calls a "computer scam".

Solmecke said he had received thousands of calls from people unsure whether to pay the fine or not. "Most of them remember having accessed RedTube.com in the past, though none of them had very clear recollections about the titles and storylines of the films they had watched."

• This article was amended on 18 December 2013 to clarify that it was the names of IP address holders which the Cologne court ordered to be released, not the IP addresses.